Yes,this is permissible.

Is it permissible to observe a nafil fast without making intention in the night?

Fasting | March 22, 2014 6:30 am
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I would like to know if somebody is repaying fast for ramadhan, can they repay the fast continuously or must they do it like every other day?

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Is it permissible for the mujahideen to break their obligatory fast to give them stregnth during the battle?